What most distinguishes humans from apes is their sex-linked genes, according to a joint research team of Korean and Japanese scientists.

The International Consortium of the Chimpanzee Genome Project said that it revealed the complete DNA coding sequence of chimpanzees’ Y chromosomes, which determines the development of sexual characteristics, and compared it with that of humans. The new findings give crucial insight into the evolution of humans, who share a common ancestor with Chimpanzees, the research team said.

According to the team, human Y chromosomes have almost three times more information than that of Chimpanzees, the closest relative to humans among animal species. It also said that the sex chromosome has made more changes within its structure than non-sex chromosomes, meaning that it has most affected the evolution of the human species.

``The difference in Y chromosomes reflects the different behaviors of humans and chimpanzees, such as their sexual culture and the size of their groups,’’ said Prof. Park Hong-seog of the Korea Research Institute of Bioscience and Biotechnology, ``And the evolutions are still underway in humans.’’

The consortium is composed of 19 scientists from Japan and Korea. The report, named ``comparative analysis of chimpanzee and human y chromosomes unveils complex evolutional pathway,’’ was published in the U.S. science journal Nature Genetics on Monday.

A chromosome is a package of DNA, which carries genetic information of each organism. Humans have 22 pairs of non-sexual chromosomes, or autosomes, and two sexual chromosomes X and Y.

Chimpanzees have 23 pairs of autosomes and two sexual chromosomes X and Y, but their Y chromosomes have only 23 million blocks of genetic information in it while the human has some 60 million blocks in its Y chromosome, meaning that humans are more complicated in sex than chimpanzees.

In 2002, the team found that humans and chimpanzees share 98.77 percent of the same genome sequence. The genetic divergence between man and the chimpanzee is believed to have taken place sometime between the last five and seven million years.

``The Y chromosome has unique characteristics compared with the autosomes or X chromosomes. Compared with humans’, chimpanzees’ Y chromosomes have a very low diversity in their DNA sequences, and this is because of the differences in the two species’ social structure,’’ the team said in a press release.

The consortium has been working on a comparison of human and chimpanzee genes since 2001. Last year it revealed the complete DNA sequence of chimpanzee chromosome 22. For its next step, the team may work on chromosome X of the chimpanzee if the budget allows, Park said.

More than half of the Y chromosomes of chimpanzees, the closest ape relatives of humans, have been decoded by a collaborative effort between Korean and Japanese researchers.

The success is considered to be a key to gaining further insights into human evolution and how immunity and infectious diseases, or cancer, occur in human and chimpanzees, respectively.

“We worked with a research team headed by Dr. Aso Fujiyama at Japan’s RIKEN research institute. Out of 23 million base pairs of chimpanzee Y chromosomes, we decoded 55 percent, or 12.7 million base pairs. We compared chimpanzee and human Y chromosomes. The comparison sheds light on the evolution of humans and chimpanzees,” said Dr. Park Hong-seok at the Korea Research Institute of Bioscience and Biotechnology on January 1.

The project received assistance from the Ministry of Science and Technology, and the research results were published in the January 2 issue of the online edition of Nature Genetics, the world’s leading journal on genetics.

Chimpanzees Do Not Inherit Immunity Diseases–

It is known that the human-chimpanzee split occurred about five to six million years ago, and each species went their separate evolutionary way.

In the process, chromosomes of both humans and chimpanzees underwent major changes. In particular, the Y chromosome, whose genes are involved in sex determination, is the chromosome that went through the most fundamental changes.

The joint research team found 19 genes on the Y chromosome of chimpanzees and compared them with the human Y chromosome (which contains 20 genes). The researchers could not locate CD24L4, a gene that is related to immunity and infectious diseases, on chimpanzee Y chromosomes.

“Human CD24L4 genes appear on the surface of cells that have immune and infectious diseases and cancer. Our discovery proves that humans and chimps contract diseases in different ways,” Park explained.

“Chimpanzees do not suffer from AIDS, dementia and malaria. Chimps have certain genes that we don’t have. By looking at them, we may find some clues for treating diseases in humans,” he added.

Different Genes, Different Sexual Behavior–

Most men are monogamous. In contrast, male chimps are polygamous. These different types of sexual behavior have had an impact on the evolution of the Y chromosome.

According to scientists, chimpanzee Y chromosomes differ from one another more widely than human Y chromosomes do. Moreover, chimpanzee Y chromosomes degenerate faster than those of humans.

“Because chimps are polygamous, a number of younger male chimps share the Y chromosomes of their father. Consequently, their Y chromosomes are similar. If a gene disappears from the Y chromosome of a male leader (degeneration), the Y chromosome of the younger generation degenerates faster,” said Park.

In January 2002, the same researchers showed that 98.77 percent of the chromosome sequences of humans and chimps match, and the result was announced in Science. In May 2004, they completely decoded chromosome 22 of chimpanzees and compared it with human chromosome 21, which has the same function as chimpanzee chromosome 22. The study was published in Nature.